Tube Amp Question

[Nathan of Brainfertilizer Fame]

What happens when a tube goes bad?

Or to put it another way:

If you plug in a guitar and have one second of full volume, and then it suddenly cuts down about about 25% or less of the volume, still sounds distorted but a little tinny, what might have gone wrong with the amp?

I think it is the amp because

1) I tried to wiggle the cable jack at both the amp and guitar ends, and ran through the length of the cable without getting even a flicker of full volume

2) I used the same cable with another amp and get full volume

3) I got the same result using two different guitars

4) I got the same result with a different, brand new cable

Thoughts?

[Steve Haynie]

There are more parts that could contribute to your problem. If you have an amp with matching tubes like a bunch of 12AX7's you can switch out the reverb tube and turn the reverb off. Use that tube to replace each of the other 12AX7's one at a time.

Capacitors and resistors can cause problems, too.

What kind of amp is giving you trouble?

[Nathan of Brainfertilizer Fame]

Peavey Classic 30.

Kind of out of the blue. I only spent 10 minutes trying to work it out, so there is still a distinct possibility the problem is operator error.

But does that sound like a tube going bad? Do they go out like a light-bulb burning out? Or is it more of a fade-out over time?

[Nathan of Brainfertilizer Fame]

The plot sickens thickens:

It doesn't take much knob turning to get back to original levels of volume.

I never really paid much attention to where the knobs were. Somewhere around "1": good enough volume to sound good, not enough to bother anyone else in the house badly. Just barely enough to feed back on some guitars.

So now it is turned up to 1.6 or so. Same level of volume as before.

It is possible, I guess, that as I grabbed the cable to plug it in to the guitar, it brushed the chickenfoot knob and pulled it toward quieter, and I just didn't notice it, or remember where the knobs were set before.

Two problems with this, though:

1) the volume had dropped on both the clean and distorted channels, by about the same amount. And turning both knobs up by about the same amount returned the volume to what I would consider "normal". It seems unlikely that the guitar cable bumped both volume knobs by the same amount without also bumping the gain knob (which is between the two volume knobs).

2) Unless it is my imagination, even at the more normal volume, there is just a slight bit of tinniness or fragility to the tone.

Or is it most likely there is nothing wrong and I'm imagining all this?

[Ralph]

Have you been hanging out with Thorn?

[tomteriffic]

If you have a spare "known good" tube (all of the preamp tubes in a Classic 30 are 12ax7's) handy, power up the amp and tap each of the tubes with the eraser end of a pencil. if any of them put a pop, ping or other untoward noise out the speaker, it's suspect. See if replacing that one with the known good tube fixes it. Similarly with the power tubes, although it's recommended to change them in pairs.

Really, though, at those lower levels, it could be just about anything. Twist all the knobs through their rotations several times. Plug a cable in and out of all of the jacks a few times. Oxidation and corrosion happen and can cause gremlins just like this.

If none of the above, the first suspect would be the input jack itself. It's attached to the circuit board in an odd way, and the circuit board itself is folded up into a "J" shape inside the chassis. Plug a cable in and wiggle it a little by plug itself. Check for crackles or cutting in/out. Don't get medieval on it, a gentle wiggle will do. You don't want to cause a problem or make a minor one worse.

[Armitage]

Peavey amps are known for dirty send receive Jacks. Give them a spray and pull a cable in and out a few times.

[Thundernotes]

^^^^^^^^ This. Marshalls are notorious for the same problem.

[Nathan of Brainfertilizer Fame]

It seems to have cleared up with a little work on the jack.

Whew.

[Roshark]

I had the same problem with a Marshall 30 watt head . It was very intermittent and hard for any teck to trouble shoot because it would never do it for them on their bench . 3 trips later to the repair shop the cause turned out to be a cold soldered joint to one of the pins of a power tube .

[crunchee]

Tube amps can fool people, there's lots of things that can go wrong with them, either singly or together. I once had a Deluxe Reverb that had had recent servicing, one night I was playing through it, and it started sounding more distorted and 'quieter' (steadily decreasing volume)...I'd shut it off, let it cool off, turn it back on, and repeated the process, and it kept getting worse until NO sound came out. Needless to say, I shut it down from further use, and eventually took it to a tech. It turned out that it was the original crummy Oxford speaker that had gone south, and nothing else. That was kinda of a surprise, as I'd only been playing through it at at 1/3 to 2/3 of it's max volume, but it was enough for the speaker to die on me. In another story, I'd accidentally put the wrong value fuse in a small practice tube amp, and it did a very similar thing...increasing distortion and decreasing volume, soundwise...but it also started to stink like burnt rubber, too. The fuse that I had put in was too big for the amp, so it kept drawing more current than it was supposed to, rather than having the correct value fuse installed, that would have kept this from happening in the first place. Bye Bye, Transformer!